The present invention relates to mobile communication devices and communication methods. More particularly, the invention relates to such a device and method that permit bi-directional communication between a mobile device and a central radio without requiring the mobile device to continuously operate its power-consuming RF front end and without the need for an accurate, power-consuming clock in the mobile device.
Wireless communication systems require some sort of multiplexing to allow several radios to simultaneously transmit and receive communications over a single communication channel. Such multiplexing may be provided by code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), and other channel access methods. These and other communication methods and protocols work well for radios that do not have power limitations, but they do not work as well for applications where the radios cannot be readily recharged because they require the radios to continuously or periodically “listen” for communications, and such listening consumes a great deal of power by the radios' RF front ends.
One prior art communication method attempts to solve this problem by synchronizing or scheduling the communications between a mobile radio and a central radio so that the mobile radio only has to turn on its RF front end and transmit and receive communications periodically according to a fixed schedule. For example, a mobile radio that is primarily used to report data to a central radio and occasionally receive instructions back from the central radio may be scheduled to turn on every 8 hours (or any other time interval). Although this reduces the power consumption of the mobile radio's RF front end, it necessitates the use of highly accurate and power-consuming clocks at both the central radio and the mobile radio to maintain the synchronization between the radios. Such clocks are expensive and further drain the battery of the mobile radio.